If a hydrocarbon bearing subterranean formation either lacks permeability or flow capacity for cost effective recovery of the hydrocarbon, then it is common practice to use hydraulic fracturing of the formation to increase the flow of the hydrocarbon, typically oil or gas. This method of stimulation creates flow channels for the hydrocarbon to escape the formation into a wellbore penetrating the formation, to maintain well production.
The wellbore typically consists of a metal pipe, commonly known as a “casing”, “production casing”, “wellbore liner” or “completion string”, which is tripped into the original (uncased) borehole and is cemented into place. Fracturing of the formation occurs when a treatment fluid is pumped under high pressure into the casing, usually via a tubular treatment string run inside the casing, and is ejected through holes in the casing, and through the cement, into the formation to cause fractures in its strata. The treatment fluid carries a proppant, such as sand or the like, which penetrates the fractures to hold them open after the treatment fluid pressure is released, and can include additives such as acids.
A current method of sealing the casing hole before fracturing begins employs a “burstable disk”, also known as a “rupture disk” or “burst disk”. The disk is formed either by machining a wall of the casing to produce a thin portion that serves as the burstable disk, or it may be a thin sheet of material placed over the casing hole. The disk has a rupture pressure threshold, and is located to block the flow of fluids through the hole while intact. Once the treatment fluid pressure reaches this threshold, the disk bursts to allow the treatment fluid to escape through the casing hole and fracture the formation strata.
A disadvantage of a burstable disk is that it merely acts as a non-reclosing pressure relief seal. The disk itself is not designed to participate in or enhance the fracturing process being performed by the pressurized fracturing fluid. A disk is in effect a membrane that either holds back a fluid, or ruptures to release the fluid. A ruptured disk is either non-fragmenting, meaning that the ruptured pieces of the disk remain attached to the perimeter of the disk, or is fragmenting, meaning that the membrane breaks into pieces which are lost.
Another disadvantage of a burstable disk is that a barrier, or cap, must be provided intermediate the disk and the wellbore annulus to protect the disk from the pressures present in the annulus. Since a differential pressure exists between the annulus and the casing, the barrier prevents pressure outside the casing from bursting the burstable disk inwardly during placement, cementing or the like. The chamber formed between the barrier and the disk must be thoroughly sealed and kept at or near atmospheric pressure until the disk is burst. If the seal is compromised and the chamber pressure changes, then the disk's rupture pressure threshold will change and could result in premature failure or untimely bursting of the disk. The disk could also malfunction should cement penetrate the chamber.
What is therefore desired is a novel downhole apparatus and insert assembly having a core which overcomes the limitations and disadvantages of the existing tools. Preferably, it should provide a means of sealing a port in a completion string from fluid flow therethrough when the insert is intact. When a threshold fluid pressure is reached, the core should disengage from the insert to provide a projectile that should impact the strata of the target formation to enhance the fracing process. The insert assembly should avoid the need for a barrier that is sealed from the annulus, but rather provide a ported debris shield that should allow annulus pressures to reach the core. The shield should form a chamber in the insert to retain a gel to obstruct entry of cement thereinto and to prevent setting of the cement which it contacts.